L-carnitine universally exists in the organs of mammal animals, and also in some plants and microorganisms. The major pharmacological effect of L-carnitine is transporting the long-chain fatty acid into the mitochondria, and thereof achieving the oxidation of fatty acid to produce energy. L-carnitine is widely used in drugs and health care products. L-carnitine was discovered in muscle extracts by Gulewitsch and Krimberg in 1905.
Natural carnitine is L-carnitine, and only L-carnitine is physiological active. L-carnitine used in current drug industry is mostly obtained by chemical synthesis. Unlike natural L-carnitine, it is difficult to obtain a totally pure L-carnitine because of the materials and synthetic route applied that D-carnitine, a dextrarotatory, is usually also obtained. D-carnitine is a competitive inhibitior of carnitine acetyl transferase (CAT) and carnitine palmityl transferase (PTC). Therefore about 10% patients suffered myasthenia gravis after taking the DL-carnitine (Martindale: the Extra Pharmacopoeia (33th): 1356). Therefore taking drug safety into consideration, it's necessary to strictly control the content of the D-carnitine in the chemical synthetic process.
Patent JP63185947 described in 1988 obtaining L-carnitine by turning a chiral epichlorohydrin to a quaternary ammonium salt, which is followed by the cyanation, hydrolysis, and demineralization reactions. It is unnecessary for this method to do the chiral separation because the starting material used for preparing L-carnitine is optically pure. However this patent mentioned neither the content of the optical isomers in the chiral materials, nor how to detect the content of L-carnitine and D-carnitine in the final product.
Patent “production of optically active quaternary ammonium salt” (JP3287567A) only described the production of the L-3-chloro-2-hydroxypropyl trimethylamine from the chiral epichlorohydrin, and mentioned the L-3-chloro-2-hydroxypropyl trimethylamine is a intermediate for optically active carnitine. The patent mentioned the optical activity of L-3-chloro-2-hydroxypropyl trimethylamine, but it did not provide the content of the optical isomers in the chiral materials nor how to detect it accurately.
The article “Synthesis of L-(−)-carnitine” (Chinese Journal of Synthetic Chemistry, vol 12, 2004) described in detail that to prepare the L-3-chloro-2-hydroxypropyl trimethylamine by amination from the chiral epichlorohydrin, then to produce the L-(−)-chloride-3-cyan-2-hydroxypropyl trimethylamine by cyanidation, and finally to obtain L-carnitine by hydrolysis, during which the specific rotation of the epichlorohydrin, the intermediate and the product was detected. However, neither the content of optical isomers in the epichlorohydrin, the intermediate and the product, nor the accurate detection was described. The method described above has quite high cost to produce pure L-carnitine. Another report about the synthesis of L-carnitine (Chinese journal of pharmaceuticals, 2006, 37(12)) provided a method that first hydrolyzing the racemic epichlorohydrin catalyzed by chiral salen-Co III complex (2) ([(R,R), N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino(2-)]cobalt acetate) to obtain S-epichlorohydrin, and then obtaining L-carnitine by amination, cyanidation, hydrolysis and ion exchange. It described only the specific rotation detection of the epichlorohydrin, the intermediate and the product, however, neither the content of optical isomers in the epichlorohydrin, the intermediate and the product, nor accurate detection was described. Moreover, the control method which is used to reduce the cost and raise the purity of L-carnitine was not mentioned.
The methods of detecting the content of D-carnitine in L-carnitine are as follows:
It was in J. Pharm. Biomed. Anal. 30 (2002) 209-218 reported detection of the content of D-carnitine in L-carnitine by combining HPLC with a derivatization reagent, the (+)-FLEC ((+)-1-(9-Fluoren)-ethyl chloroformate). The detection of the content of D-carnitine in the product L-carnitine was reported, however, nothing about detecting the content of enantiomer in the starting material and intermediate during the preparation of L-carnitine was reported, nor the relative control method that could be used to reduce the cost and raise the purity of L-carnitine.
Because the S-epichlorohydrin is obtained from organic synthesis or biological conversion as well, detection of the optical rotation without accurate detection of its content of the optical isomer may bring some optical isomer impurity in each step of the preparation of L-carnitine. Because there is no chiral separation during the whole preparation, the final product may possibly contain the optical isomer impurity. Furthermore, many steps are involved in the preparation of L-carnitine, so that the racemization may easily happen, Therefore accurate detection and control of the content of isomer of the starting material and accurate detection the optical purity of the intermediate in each step is necessary to ensure if racemization which affects the optical purity of the final product occurs.
Because the dextroisomer of L-carnitine is harmful to human body, it is necessary to find an effective method to make sure obtaining L-carnitine with high optical purity by detecting the optical purity of the starting materials and the intermediates and controlling the content of the optical isomer impurity in each step. This is important for ensuring human health and improving the purity of synthetic L-carnitine. Besides, concerning about the environment protection, to ensure the yield of each synthetic process and to avoid an unaccepted intermediate entering into the next reaction during the multi-synthetic steps decreases the polluting steps and reduces the cost of the “three wastes” treatment. It is also important to energy saving and emission reduction.